This non-provisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 2001-156808 filed in Japan on May 25, 2001, which is herein incorporated by reference.
1. Field of the Invention
This invention relates to a failure diagnostic system that determines whether an evaporated fuel processing system for use in preventing evaporated fuel generated in a fuel tank from being emitted into the air has failed or not.
2. Description of Related Art
Japanese Laid-Open Patent Publication (Kokai) No. 2000-161150 discloses the method comprising the steps of reducing an internal pressure of a fuel tank to a predetermined negative pressure, then sealing off the fuel tank from external air, and monitoring the degree of increase in the internal pressure of the fuel tank to determine that an evaporated fuel processing system has failed if an increase equal to or greater than the predetermined value is detected. In the case where this method is adopted, if fuel is sloshing in the fuel tank, there is the possibility of a false diagnosis due to a great change in the tank internal pressure.
Japanese Laid-Open Patent Publication (Kokai) No. 6-159157 discloses the method comprising the steps of leading negative pressure into a fuel tank for a predetermined period of time and determining whether an evaporated fuel processing system has failed if the tank internal pressure does not become equal to or lower than a predetermined value. If a change xcex94P in the tank internal pressure is equal to or greater than a predetermined value, it is determined that fuel is sloshing in the fuel tank and the diagnosis is stopped. If the tank internal pressure becomes lower than a pressure Ps detected prior to the determination that the fuel is sloshing, the diagnosis is resumed. Therefore, it may be considered that the above-mentioned problem may be solved by applying the method disclosed in the Japanese Laid-Open Patent Publication (Kokai) No. 6-159157 to the method disclosed in the Japanese Laid-Open Patent Publication (Kokai) No. 2000-161150.
In the method disclosed in the Japanese Laid-Open Patent Publication (Kokai) No. 2000-161150, however, a pressure restoration status after the pressure decrease is monitored. During the monitoring, the tank internal pressure is likely to gradually increase even when the evaporated fuel processing apparatus is normally operating. Therefore, if the method in which the diagnosis is not resumed until the detected pressure becomes equal to or lower than a pressure detected prior to the rapid increase in the pressure as disclosed in the Japanese Laid-Open Patent Publication (Kokai) No. 6-159157 is applied to the method disclosed in the Japanese Laid-Open Patent Publication (Kokai) No. 2000-161150, it is impossible to resume the diagnosis, and the diagnosis is stopped whenever a rapid increase in the pressure is detected. This considerably decreases failure diagnosis opportunities.
It is therefore an object of the present invention to provide a failure diagnostic system that is capable of correctly determining whether an evaporated fuel processing system has failed or not without considerably decreasing diagnosis opportunities even if the internal pressure of a fuel tank is rapidly increased due to sloshing of fuel or the like.
To attain the above object, the present invention provides a failure diagnostic system, which reduces a pressure in a fuel tank to a predetermined negative pressure, seals off the fuel tank from external air, and then determines whether an evaporated fuel processing system has failed or not according to the degree of increase in the pressure in the fuel tank, compares the pressure in the fuel tank with a reference value that is increased at a predetermined rate, and stops update of update pressure if the pressure has become higher than the reference value and resumes update of update pressure if the pressure has become equal to or lower than the reference value.
With this arrangement, if the detected pressure in the fuel tank has become greater than the reference value that is increased at a predetermined rate, the update of the detected pressure is stopped. This prevents false diagnosis in the case where the internal pressure of the fuel tank is rapidly increased to sloshing of the fuel or the like, thus enabling accurate diagnosis. After the rapid increase in the internal pressure of the fuel tank during a pressure restoring process after the pressure reduction, the detected pressure becomes equal to or lower than the reference value before it becomes equal to the pressure detected prior to the rapid increase, because the reference value is increased at the predetermined rate. If the detected pressure becomes equal to or lower than the reference value, the update of the detected pressure is resumed. This enables an improvement in diagnostic accuracy without considerably decreasing diagnosis opportunities.
It is preferred that while the pressure is higher than the reference value, the update pressure updating device regards an update pressure before the pressure becomes higher than the reference value as the update pressure.
This surely prevents false determination without considerably decreasing diagnosis opportunities.
Further, the detected pressure may be an output itself from a detecting device that detects the internal pressure of the fuel tank, but an output from the detecting device may be processed through a filter to be used as the detected pressure. In the case where the output processed through the filter is used as the detected pressure, detecting errors or small variations in outputs from the detecting device are averaged by the filter, and only great variations exceeding the permissible amount of the filter are compared with a reference value. This assures reliable diagnostic performance.